This invention relates to a liquid-phase process for catalytically isomerizing a xylene stream containing a minor amount of ethylbenzene to a mixture rich in paraxylene, and, more particularly, to a liquid-phase process to catalytically isomerize a xylene stream containing a minor amount of ethylbenzene to a product mixture rich in para-xylene over a silica-supported, gallium-containing, crystalline silicate molecular sieve in which substantially all of the gallium is in molecular sieve lattice positions or is intimately associated with the silicate sieve crystal lattice.
Typically, p-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by isomerization followed by, for example, lower-temperature crystallization of the p-xylene with recycle of the crystallizer liquid phase to the isomerizer. Principal raw materials are catalytically reformed naphthas and petroleum distillates. The fractions from these sources that contain the C.sub.8 aromatics vary quite widely in composition but will usually contain 10 to 35 weight percent ethylbenzene and up to about 10 weight percent primarily C.sub.9 paraffins and naphthenes with the remainder being primarily xylenes divided approximately 50 weight percent meta, and 25 percent each of the ortho and para isomers. Feeds that do not have the primarily C.sub.9 paraffins and naphthenes removed by extraction are termed "unextracted" xylene feeds.
The ethylbenzene in a xylene mixture is very difficult to separate from the other components due to similar volatility, and, if it can be converted during isomerization to products more readily separated from the xylenes, build up of ethylbenzene in the recycle loop is prevented and process economics are greatly improved. The primarily C.sub.9 paraffins and naphthenes present in unextracted feeds unless removed also build up in the recycle loop and are usually extracted prior to isomerization as most commercial isomerization processes do not provide a catalyst which effectively converts them to easily separable-by-distillation products.
The xylene isomerization process is an important step in the eventual production of polyester based upon terephthalic acid. The p-xylene isomerization product is oxidized to terephthalic acid, conveniently by a cobalt ion/acetic acid liquid-phase oxidation, which is a raw material for the production of polyethylene terephthalate.
Xylene isomerization is commercially performed in the vapor phase using a catalyst that exhibits catalytic activity for both the isomerization of xylenes and the conversion of at least the ethylbenzene impurity present in the feedstock as mentioned above. Isomerization in the liquid phase, however, is preferred, but a number of problems, mainly catalyst lifetime, arising in liquid-phase isomerization have led to the vapor-phase process being the commercial choice.
Isomerization of xylenes in the liquid phase has been a subject of study by a number of workers. See for example, U.S. Pat. Nos. 3,777,400; 3,856,871; 4,268,420; and 4,269,813; Japanese Kokai 57-32233 (1982); and a 1972 article in "Hydrocarbon Processing" at p. 85 by P. Grandio, F. H. Schneider, A. B. Schwartz and J. J. Wise. In these reports, the primary reaction observed was isomerization of xylenes, even in the presence of ethylbenzene and other impurities. In fact, molecules such as toluene and ethylbenzene were sometimes added to the process to improve the selectivity of the catalysts toward isomerization (rather than disproportionation) of the xylenes. Most of the catalysts which were employed in the above works contained zeolites of the large-pore type, e.g., faujasite-type zeolites or mordenite.
U.S. Pat. No. 3,856,879 is an early report of the use of shape-selective, molecular-sieve-containing catalysts for the isomerization of xylenes and the conversion of ethylbenzene in the liquid phase. The aluminosilicate zeolites ZSM-5, ZSM-12, and ZSM-21 are recommended for use in this process. Although by-product distributions are not reported in the patent, the xylene feed is said to be isomerized to its equilibrium isomer concentrations and the ethylbenzene converted via the transalkylation and disproportionation mechanisms. A catalyst containing the aluminosilicate molecular sieve, ZSM-5, was claimed to exhibit no deactivation, even in the absence of hydrogen. However, other unpublished results show that, contrary to the teaching of this patent, a ZSM-5 type sieve (35 weight percent on alumina) catalyst composition exhibits rapid deactivation and poor selectivity.
Gallium-containing molecular sieve catalyst compositions are taught in a number of publications for hydrocarbon conversions; in particular, U.S.S.N. 202,210 filed on June 3, 1988, now U.S. Pat. No. 4,812,536, teaches the use of such materials with added magnesium compound for the para-ethylation of toluene.
Now catalyst compositions based upon gallium silicate molecular sieves have been found which exhibit for the liquid-phase isomerization of xylene streams containing a minor amount of ethylbenzene improved catalyst lifetimes at good selectivities as measured by (1) considerably better overall selectivity as measured by the ratio of percent total ethylbenzene conversion to percent xylene loss, and (2) a lower deactivation rate (longer lifetime) than the usual commercial aluminosilicates and borosilicates when used in liquid-phase xylene isomerization.